Full understanding of many biologically important processes requires detailed knowledge of membrane functions. One area in which information is sparse is the specific interaction of transport proteins with cell membranes to effect the entry of a small molecule. Examples of such transport systems are transferrin-iron and hemopexin-heme. These systems have essential functions and aberrations in them can be deleterious. Hemopexin, a serum beta-glycoprotein, functions in the selective transport of intravascular heme to the liver and thus plays a role in conserving iron as well as in preventing toxic effects of heme. The heme-hemopexin-liver cell system is an informative model of transport protein-ligand-cell interactions because of the specificity of the interaction of hemopexin with the liver, the information available on the chemistry and biology of hemopexin, and its analogy with other protein-mediated transport processes. Hemopexin's function is being studied in several ways. First, the interaction of heme-hemopexin with the liver in vivo and liver cells and liver cell membranes in vitro are being characterized. Questions of interest include: How does the heme-hemopexin complex interact with the hepatocyte? What is the nature of the recognition site on the membrane for the complex? Second, we are studying the fate of heme and hemopexin inside the liver cell, with particular emphasis on the role of subcellular organelles in the intracellular transport of heme to microsomal heme oxygenase.